ABSORPTION OF PRODUCTS OF DIGESTION. 39 



Fleshy fibres, forming a sphincter, constantly in action, sur- 

 round the aperture of the bowel, and thus prevent the escape 

 of its contents, until the moment arrives for its evacuation. 

 This is effected mainly by the muscular action of the bowel 

 itself, assisted occasionally by the diaphragm and levatores 

 ani muscles. 



74 Theory of Digestion. The object of digestion is to 

 fit the food for absorption by the lacteals. A portion of the 

 alimentary mass being soluble in water, is of consequence 

 directly dissolved by the saliva, the gastric juice, and the 

 water swallowed as part of our food, without the intervention 

 of any special active principle. The animal diastase con- 

 tained in the saliva possesses the property of transforming 

 fecula into glucose, and thus determines the solubility of a 

 portion of the amylaceous matters introduced into the stomach. 

 The pepsin contained in the gastric juice acts in an analogous 

 manner upon the fibrin, albumen, &c., and liquefies these sub- 

 stances in the cavity of the stomach. The fecula, which may 

 have resisted the action of these agents, and has reached the 

 intestine untouched, there meets with the pancreatic fluid, 

 whose action is analogous to that of the salivary secretions, 

 and thus the solution of the amylaceous portions of the food 

 is completed. Finally, the fatty matters, which also have 

 escaped the action of the saliva and gastric juice, are formed 

 into emulsions by the action of the pancreatic secretion, and 

 sometimes by the aid of the alkali contained in the bile ; and 

 just as these various solutions go on, the product is taken up 

 by the absorbents in connexion with the walls of the stomach 

 and intestine. Certain of the substances thus dissolved under- 

 go modifications in their chemical nature. Thus the sugar of 

 the sugar-cane is changed into glucose ; but the most impor- 

 tant and most general of the phenomena connected with 

 digestion, is the liquefaction of the alimentary matters. 



Absorption of the Products of Digestion. 



75. The nutritive matter thus extracted from the ali- 

 ment has now to pass into the mass of blood, which fluid it 

 is destined to renovate. 



Some of the liquids and soluble matters introduced into 

 the stomach and intestine are absorbed directly by the veins, 

 which abound in the walls of these organs ; but the greater 

 part of the fibrin and fatty matters which form the chyle 



